Process for enameling aluminum



United States Patent FOR ENAMELING 7 a ms-AC This invention relates to a process for producing porcelain or vitreous enamel coatings on aluminum. As used herein, the term aluminum includes aluminum of varlousgracles and also aluminum base alloys.

Heretofore various processes'have been proposed for applying vitreous enamel coatings to aluminum. However, it has beenfound that the enamels do notadhe're tenaciously 'to the metal surface particularly in the presenceof water and moisture. In fact, it is in connection with aluminum alloys which are ordinarily desiredfor structural purposes that the adherence of the enamels to the metal surface has been least satisfactory. The particular difiiculty encountered is referred to as waterspalling and is evidenced'by a flaking'olfof the enamel.

Various methods of pretreating the metal have been proposed for reducing and overcoming this objectionable tendency.

One proposal involves immersing the aluminum in an aqueous alkaline solutionofchromate, prefiring to temperatures between 350 and t 600 F. and thereafter enameling. This procedure requires careful control of the solution composition andoperating conditions for the alkaline chromate treatment, along with careful control of the prefiring step which is, of course, costly. Nevertheless, the procedure has not assured the production of enameled aluminum free'from more than a reasonable number of failures attributable to water-,spalling. In fact our experience with this procedure has been that failures are-too often encountered,occasionally amounting to 50% of the material enameled.

Another procedure proposed for reducing a waterspalling of enameled aluminum alloys includes a chemical treatment of the surface to be enameled in a phosphateacid-bath, also followed by a prefiring step: prior to the enameling step. We have found, however, that the bath is rapidly exhausted and requires rather close control. Consequently, we have encountered failures with this procedure as well.

Another procedure proposed for reducing water-spalling of enameled aluminum alloys and designed to eliminate the expensive time-consuming step of prefiring comprises treating the aluminum in an aqueous acid solution containing between 2 and 4% by weight of chromic acid and between and 25% sulfuric acid, at a temperature of between 70 and 100 C., for. a period between one and five minutes, and thereafter enameling the aluminum. While in some instances desirable resultshave been obtained byusing this process, this processhas not proved satisfactory without the imposition of a prefiring step between the treatment with the mixture ofchromic acid and sulfuric acid and the enameling step. This is particularly true when relatively pure aluminum alloys such as SS arebeing enameled.

Accordingly, it is an object of the present invention to provide a process for applying a vitreous enamel coating to aluminum.

2,858,245 Patented Oct 28 1958 2 Y j It is a principal object of the invention to provide a process for enameling of aluminum which obviates the costlyzand hardato control step of prefiring. A still advantages will appear in the ensuing description of the invention.

These objects and advantages are achieved by the method of producinga non-water-sp'alling vitreous enamel coating on aluminum and aluminum alloys which comprises subjecting the metal surface to be enameled to" a sequential series ofoperat ions to clean and pretreat said surface before applying and firing the enamel coating, said sequential operations comprising first subjecting said surface to the action deoxidizing solution capable of dissolving metallic oxides, and then forming a chromate conversion coating on said surface by subjecting it to the action of an aqueous acid solution containing hexavalent chromium .ions and fluoride ions, rinsing the surface to remove the retreating solution, and thereafterapplying the enamel to the thus treated-surface and forming the vitreous enamel coating byfusing the enamel,

The invention therefore involves a process for producing an adherent porcelain o r vitreous enamel coating which comprises four steps: cleaning, deoxidizing, chromate coating and 'enameling.

In the first operation, surface contamination suchaas dirt, grease, oils andthe like, is removedby ele aning the'surface' with an alkali this operation the metal should be cleaned in such a manner as to avoid deeply etching the surface, since severe etching will lead to inferior adhesion of the enamel to the base .metal. by employing a mild alkali solution and preferably one containing an inhibitor such as trisodium phosphate which reduces the tendency of the alkali solution to dissolve and etch aluminum surfaces. .In'hibited alkali solutions of an aqueous alkali solution; thereafter subjecting said surface to the action of a cleaning solution. During This can be accomplished having a pH below 12 have been found to be particularly suitable for, cleaning aluminum surfaces. Solvent clean ing of the metal is not suitable for the purposes of this invention. If a solvent cleaner is employed the results finally obtained will be markedly inferior to the vitreous enamel coatings formed on alkali cleaned surfaces.

After alkali cleaning, it is preferred to remove the residue of cleaning solution from the surface by means of a water rinse. action of the alkali and also prevent sequently applied deoxidizing solution.

In the second step, the aluminum surface is deoxidized oxides adhering thereto. In general by removing metallic This will tend to arrest the etching killing" the subit has been found that any conventional deoxidizing solu tion for aluminum may be used inlthis step. However,

in the process of the present invention it has been found that deoxidation can best. be accomplished by means of dilute phosphoric acid, i. e., from 4 to 12% by volume of phosphoric acid. If phosphoric acid is'notused;

materials such astniti'ic acid and mix? chromic acid may be used; In general,"

other deoxidizing tures of nitric and treatment, should also be of such a nature as not to severely etch the surface for the reasons given above in regard to step 1.

is customary to rinse thedeoxidized order to remove any adhering After deoxidationjit surface withzwaterih deoxidizing solution," and prevent carrying it over into the chromate coating bath,

In the third step, the clean and deoxidized surface is subjected to the action of an acidic chromate coating solution containing hexavalent chromium ions and fluoride ions in proportions which produce a chromate conversion film on the surface. Such solutions and the application thereof. are well known in the metal coating art and the solutions are sold commercially in the United States under various trade names such as Alodine 600, Alodine 1200, Bonderite 710, Iridite 14, etc. Additionally, solutions such as those disclosed in United States Patents, such as 2,276,353 and 2,507,956 are suitable for this operation.

After the chromate conversion coating has been formed on the surface of aluminum, it is essential to remove substantially all of the adhering coating solution from the surface before applying the enamel. In general, this removal may be done by thoroughly rinsing the coated surface with water. After removal of the adhering coating solution from the coated surface, it is also preferred that the surface be dried.

After drying, the surface may be coated with enamel which is applied and baked in accordance with the manufacturers directions. United States Patents Nos. 2,467,- 114, 2,642,364 and 2,653,877 disclosed enamel compositions and methods of performing the enameling step per se that may be employed in the process of this invention. The enamel compositions disclosed in these patents are suitable for the enameling of aluminum, but the invention is not confined to the use of these specific compositions.

Obviously, if multicoats of enamel are to be applied, the second or third coat will be applied in the normal fashion.

As examples of how the new process may be carried out, the following non-limiting examples may be cited:

Formula No. I

Pounds Soda ash 0.35 Trisodium phosphate 0.55 Caustic soda 0.10

Formula No. I is an excellent inhibited mild alkali cleaner which when dissolved in water to give a concentration of approximately 9 oz./gal. will clean aluminum in a power spray washer in from 5 to 7 seconds without imparting objectionable etch to the surface.

After cleaning the aluminum, it is customary to rinse the surface before deoxidizing so that the alkali will not be dragged into the deoxidizing bath of which the following is a typical example:

Formula No. II

Gallons 75% phosphoric acid (H PO 0.08 Water, to make 1.00 gallon.

Formula N0. IV

Potassium dichromate grams 18 30% fiuosilicic acid ml 20 Potassium ferricyanide grams 5 Water, to make 1 liter.

Formula No. V

Water, to make 1 liter.

The chromate conversion coating may be formed from these solutions by subjecting the aluminum surface to the action of the solutions in the conventional and well known fashion, i. e., flowing, dipping, spraying, etc. After the coating is formed, residues of the coating solution are removed by thorough rinsing. After rinsing, the coated aluminum should preferably be dried.

The coated aluminum is enameled with any porcelain or vitreous enamel having a melting point below the melting point of aluminum and preferably those which form the enamel coat at temperatures below the heat tolerance of the metal, i. e., temperatures which will not adversely affect the strength of the metal. Best results have, however, been obtained by the use, at least for the first coat, of enamels containing lead, especially those of the type disclosed in the patents referred to above. Such enamels contain:

10 to 18 mole percent PbO 38 to 65 mole percent SiO;; 5 to 12 mole percent U 0 0 to 22 mole percent Na O 0 to 20 mole percent K 0, and 0 to 11 mole percent TiO the total alkali metal oxide content of the enamel being between 25 and 36 mole percent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the enamel to the total alkali metal oxide content of the enamel being between 1.8 and 3.0, all of said oxides being melted into the enamel composition. However, ceramic pigments, refractory materials and other constituents also may be contained in the enamel composition.

In the enameling step, enamel is applied to the aluminum in a conventional manner (by spraying on or dipping the aluminum in an aqueous slip and drying at low temperature, as for example) and then fired above the fusing point of the enamel at a temperature between 500 and 580 C., in some cases preferably at a temperature between 500 and 540 C., for a period between 2 and 30 minutes. Additional coats of enamel may be provided by repeating the enameling step.

It has been found that the process just described to be especially effective in the enameling of 38 aluminum alloy surfaces. It is useful and effective in the enameling of other aluminum surfaces, including the surfaces of aluminum alloys such as 525 and 618. The alloying ingredients nominally contained in these aluminum alloys are tabulated below for convenient reference:

Mn Mg Cu S1 Fe Zn Cr Ti The effectiveness of the process for producing an adherent enamel coating can, of course, be evaluated by means of the generally used water-spalling test which consists in immersing the test sample in 5 percent ammonium chloride solution for 24 to 96 hours. Numerous specimens treated in accordance with the disclosure given above have been evaluated in this test with few or no failures, even after 144 hours, thus clearly illustrating the advantages of the present invention.

We claim:

1. The method of applying vitreous enamel coatings to aluminum and aluminum alloys which comprises subjecting the metal surface to be enameled to a sequential series of operations to clean and pretreat said surface before applying and firing the enamel coating, said sequential operations comprising first subjecting said surface to the action of an aqueous alkali solution, thereafter subjecting said surface to the action of a deoxidizing solution capable of dissolving metallic oxides, and then forming a chromate conversion coating on said surface by subjecting it to the action of an aqueous acidic solution containing hexavalent chromium ions and fluoride ions, rinsing the surface to remove the treating solution, and thereafter applying the enamel to the thus treated surface and forming the vitreous enamel coating by fusing the enamel.

2. The method of applying vitreous enamel coatings to aluminum and aluminum alloys which comprises first subjecting the metal surface to be enameled to the action of a mild aqueous alkali solution containing an etch inhibitor, then treating said surface by subjecting it to the action of a dilute acid solution capable of dissolving metal oxides adhering to the metal surface without objectionably etching the metal, forming a chromate conversion coating on the thus treated surface by contacting the surface with an aqueous acidic solution containing hexavalent chromium ions and fluoride ions, rinsing the surface to remove the treating solution, and thereafter applying an enamel forming material to the thus treated surface and forming the vitreous enamel by fusing the enamel at elevated temperatures.

3. The method according to claim 2 wherein the metal surface is subjected to water rinses intermediate the various treatment steps prior to the application of the chromate conversion coating.

4. The method according to claim 2 wherein the metal, at all times prior to the application of the enamel forming material, is maintained at temperatures below the boiling point of the various treating solutions.

5. The method of applying vitreous enamel coatings to aluminum and aluminum alloys which comprises cleaning the metal surface to be enameled by subjecting it to the action of a mild alkali solution for a time suflicient to remove dirt and grease without objectionably etching the metal, rinsing the thus treated surface with water, subjecting the cleaned metal surface to the action of a deoxidizing solution having a deoxidizing action equivalent to that provided on treating the metal for less than one minute with an aqueous phosphoric acid solution containing from 4 to 12% by volume of 75% phosphoric acid, rinsing the thus treated surface with water, applying a chromate conversion coating by subjecting the thus tteat ed metal surface to the action of an acidic aqu ous solution containing hexavalent chromium ions and fluoride ions, rinsing the thus treated surface with water and thereafter applying an enamel forming material to the thus treated surface and forming the vitreous enamel coating by fusing the enamel forming material at elevated temperatures.

6. The method of applying vitreous enamel coatings to aluminum and aluminum alloys which comprises spraying the surface to be enameled with an aqueous solution containing 9 oz./gal. of a mixture of soda ash, trisodium phosphate and caustic soda (the weight percentage of such ingredients being 35%, 55% and 10% respectively) for 5 to 7 seconds, rinsing the thus treated surface with water, then spraying said surface with a phosphoric acid solution containing 0.08 gallon of phosphoric acid in suflicient water to make 1 gallon of solution, rinsing the thus treated surface with water, applying a chromate conversion coating by subjecting the thus treated metal surface to the action of an acidic aqueous solution containing hexavalent chromium ions and fluoride ions, rinsing the thus treated surface with water and thereafter applying an enamel forming material to the thus treated surface and forming the vitreous enamel coating by fusing the enamel forming material at elevated temperatures.

7. In the process for producing vitreous enamel coating on aluminum and aluminum alloys involving cleaning the surface to be enameled to remove dirt, grease, oil and corrosion products, applying a chromate coating to the cleaned surface, prefiring the chromate coated material at elevated temperatures and thereafter enameling the chromate coated surface by applying thereto an enamel forming material which on firing at temperatures below the heat tolerance of the metal but above the fusing point of the enamel forms a vitreous enamel coating, the improvement whereby the prefiring of the chromate coated material can be eliminated, characterized in that cleaning the surface to be enameled by subjecting it to the action of an alkali solution containing an etch inhibitor, deoxidizing the cleaned surface by subjecting it to the action of a conventional aluminum deoxidizing solution, applying a chromate conversion coating to the cleaned deoxidized surface by subjecting it to the action of an aqueous acidic solution containing hexavalent chromium ions and fluoride ions, and rinsing and drying the thus treated surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,137,988 Hempel Nov. 22, 1938 2,507,956 Bruno et al. May 16, 1950 2,544,139 Deyrup et al Mar. 6, 1951 2,768,103 Schuster et al. Oct. 23, 1956 FOREIGN PATENTS 919,450 Germany Oct. 21, 1954 

1. THE METHOD OF APPYING VITREOUS ENAMEL COATINGS TO ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES SUBJECTING THE METAL SURFACE TO BE ENAMELED TO A SEQUENTIAL SERIES OF OPERATIONS TO CLEAN AND PRETREAT SAID SURFACE BEFORE APPLYING AND FIRING THE ENAMEL COATING, SAID SEQUENTIAL OPERATIONS COMPRISING FIRST SUBJECTING SAID SURFACE TO THE ACTION OF AN AQUEOUS ALKALI SOLUTION, THEREAFTER SUBJECTING SAID SURFACE TO THE ACTION OF A DEOXIDIZING SOLUTION CAPABLE OF DISSOLVING METALLIC OXIDES, AND THEN FORMING A CHROMATE CONVERSION COATING ON SAID SURFACE BY SUBJECTING IT TO THE ACTION OF AN AQUEOUS ACIDIC SOLUTION CONTAINING HEXAVALENT CHROMIUM IONS AND FLUORIDE IONS, RINSING THE SURFACE TO REMOVE THE TREATING SOLUTION, AND THEREAFTER APPLYING THE ENAMEL TO THE THUS TREATED SURFACE AND FORMING THE VITREOUS ENAMEL COATING BY FUSING THE ENAMEL. 